Quantitative linear independent vector based method (QLIVBM) for image alignment

The present invention generally relates to methods and systems for image alignment to a reference location for specimens such as wafers and reticles. Certain embodiments described herein are related to quantitative linear independent vector based methods (QLIVBM) for image alignment that is particularly suitable for non-optimal sites and false alignment avoidance.

The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.

Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.

One important step that can cause an inspection to fail if not performed correctly and accurately is aligning test and reference images to each other. For example, many inspection processes subtract a reference image from a test image and then apply some defect detection method to the resulting image, often referred to as a difference image. If the images are not properly aligned to each other prior to the subtraction, then many differences between the images caused by poor alignment can be erroneously detected as defects.

Much research and development in inspection processes has therefore been directed towards developing accurate and efficient methods for aligning test and reference images to each other. One widely used, existing method involves selecting a single alignment site for wafer alignment in the setup phase. The setup phase also includes recording the alignment site kernel image in the recipe for the inspection process.

During runtime, such methods include aligning the single alignment site kernel and runtime alignment site images flowing through one of one or more alignment algorithms. The method may also include determining if the kernel image matched to the runtime alignment site image. If the kernel and runtime alignment site images do not match, the method may include determining if all alignment algorithms have been tried. If not, then alignment may be performed for the same images with another alignment algorithm. If the kernel and runtime images still do not match and the method determines that all alignment algorithms have been tried, the method may abort the alignment attempts. If the kernel and runtime images do match, then the method may determine that alignment has been achieved.

There are, however, a number of disadvantages to many of the currently used methods for image alignment. For example, in the method described above, false alignment and low die corner accuracy determination are possible. Some attempts to prevent that include trying to make the inline runtime wafer's alignment site look more like the alignment site saved in kernel during setup. Algorithm development can also involve trying to make the algorithm less sensitive (where the algorithm does more) or more sensitive (so the algorithm does less). For example, an algorithm that “does more” may do more image processing, such as emphasizing certain patterns, increasing the contrast, filtering out certain frequencies, extracting certain patterns, etc. to make runtime alignment site images look more similar to the setup alignment site image thereby making the tool less sensitive to differences between runtime and setup images and increasing the chance of achieving alignment. However, a less sensitive algorithm can lead to false alignment and premap and pixel-to-design alignment (PDA) interruption. In one such example, all of the image processing described above that is designed to make the algorithm less sensitive to alignment site image variations can actually change the runtime alignment site image so much that it aligns to the wrong setup alignment site image thereby causing false alignment and premap and PDA interruption. In the same way, a more sensitive algorithm can cause failed alignment interruption due to variations between the runtime and setup alignment target images that are not corrected by the algorithm. There is therefore a dilemma for current algorithm development on false alignment and failed alignment.

To summarize then, if alignment algorithms are less sensitive, the alignment site tends to match to the wrong location, which causes false alignment and subsequent premap and PDA interruption. However, if the alignment algorithms are too sensitive, then the alignment can fail frequently due to process change, which causes instant alignment interruption. There is also no current solution for aligning runtime wafers with missing or damaged alignment sites.

Accordingly, it would be advantageous to develop systems and methods for aligning images of a specimen to a reference location in the images that do not have one or more of the disadvantages described above.

The following description of various embodiments is not to be construed in any way as limiting the subject matter of the appended claims.

One embodiment relates to a system configured for image alignment. The system includes an imaging subsystem configured to generate images of a specimen. The system also includes a computer subsystem configured for separately aligning candidate alignment target images in the images generated for the specimen to corresponding setup images for setup alignment targets selected to have mutually linearly independent vectors between locations of the setup alignment targets and a reference location. For each of one or more of the candidate alignment target images successfully aligned to its one of the corresponding setup images, the computer subsystem is configured for separately determining coordinates of the reference location in the images generated for the specimen from coordinates for each of the one or more of the candidate alignment target images and its corresponding one of the mutually linearly independent vectors. In addition, the computer subsystem is configured for determining final coordinates of the reference location in the images generated for the specimen from the separately determined coordinates of the reference location. The system may be further configured as described herein.

Another embodiment relates to a computer-implemented method for image alignment. The method includes the separately aligning, separately determining coordinates, and determining final coordinates steps described above, which are performed by a computer subsystem. Each of the steps of the method may be performed as described further herein. The method may include any other step(s) of any other method(s) described herein. The method may be performed by any of the systems described herein.

Another embodiment relates to a non-transitory computer-readable medium storing program instructions executable on a computer system for performing a computer-implemented method for image alignment. The computer-implemented method includes the steps of the method described above. The computer-readable medium may be further configured as described herein. The steps of the computer-implemented method may be performed as described further herein. In addition, the computer-implemented method for which the program instructions are executable may include any other step(s) of any other method(s) described herein.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and are herein described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

Turning now to the drawings, it is noted that the figures are not drawn to scale. In particular, the scale of some of the elements of the figures is greatly exaggerated to emphasize characteristics of the elements. It is also noted that the figures are not drawn to the same scale. Elements shown in more than one figure that may be similarly configured have been indicated using the same reference numerals. Unless otherwise noted herein, any of the elements described and shown may include any suitable commercially available elements.

In general, the embodiments described herein are configured for image alignment. Certain embodiments relate to quantitative linear independent vector based methods (QLIVBM) that are particularly useful for non-optimal sites alignment solutions and false alignment avoidance.

The measurement of mean number of wafers ran between interruptions (MWBI) value is to evaluate the reliability and usability of inspection platforms during runtime, which can have significant financial implications. Alignment failure is the major root cause for most interruptions, and it can severely impact the MWBI value. As a consequence, the adoption of inspection platforms to high volume research and development fab utilization, as well as worldwide high-volume manufacturing applications, becomes less likely as the probability of alignment failure increases.

In currently used methods on current wafer inspection tools, if alignment algorithms are less sensitive, the alignment site tends to match to the wrong location, which causes false alignment and subsequent premap and pixel-to-design alignment (PDA) interruption. In addition, if alignment algorithms are more sensitive, the alignment fails frequently due to process change, which causes the runtime images to vary from the setup images thereby causing instant alignment interruption. There is also no solution to align runtime wafers with missing or damaged alignment sites.

The QLIVBM embodiments described herein have a number of important advantages over the currently used methods. One such advantage is that they can reduce, and even eliminate, false alignment during runtime on wafer inspection platforms. Another advantage is that they can accomplish substantially high confidence wafer alignment and die corner determination during runtime on wafer inspection platforms even if the alignment site is non-optimal, including severely damaged or even completely missing sites.

In some embodiments, the specimen is a wafer. The wafer may include any wafer known in the semiconductor arts. Although some embodiments may be described herein with respect to a wafer or wafers, the embodiments are not limited in the specimens for which they can be used. For example, the embodiments described herein may be used for specimens such as reticles, flat panels, personal computer (PC) boards, and other semiconductor specimens.

One embodiment relates to a system configured for image alignment. One embodiment of such a system is shown in. The system includes imaging subsystemconfigured to generate images of a specimen. The imaging subsystem includes and/or is coupled to a computer subsystem, e.g., computer subsystemand/or one or more computer systems.

The terms “imaging system” and “imaging subsystem” are used interchangeably herein and generally refer to the hardware configured for generating images of a specimen. In general, the imaging subsystems described herein include at least an energy source and a detector. The energy source is configured to generate energy that is directed to a specimen. The detector is configured to detect energy from the specimen and to generate output responsive to the detected energy.

In a light-based imaging subsystem, the energy directed to the specimen includes light, and the energy detected from the specimen includes light. For example, as shown in, the imaging subsystem includes an illumination subsystem configured to direct light to specimen. The illumination subsystem includes at least one light source, e.g., light source. The illumination subsystem is configured to direct the light to the specimen at one or more angles of incidence, which may include one or more oblique angles and/or one or more normal angles. For example, as shown in, light from light sourceis directed through optical elementand then lensto specimenat an oblique angle of incidence. The oblique angle of incidence may include any suitable oblique angle of incidence, which may vary depending on, for instance, characteristics of the specimen and the defects to be detected on the specimen, the characteristics of the specimen to be measured, etc.

The illumination subsystem may be configured to direct the light to the specimen at different angles of incidence. For example, the imaging subsystem may be configured to alter one or more parameters of one or more elements of the illumination subsystem such that the light can be directed to the specimen at an angle of incidence that is different than that shown in. In one such example, the imaging subsystem may be configured to move light source, optical element, and lenssuch that the light is directed to the specimen at a different oblique angle of incidence or a normal (or near normal) angle of incidence. The illumination subsystem may have any other suitable configuration known in the art for directing the light to the specimen at one or more angles of incidence sequentially or simultaneously.

The illumination subsystem may also be configured to direct light with different characteristics to the specimen. For example, optical elementmay be configured as a spectral filter and the properties of the spectral filter can be changed in a variety of different ways (e.g., by swapping out one spectral filter with another) such that different wavelengths of light can be directed to the specimen at different times.

Light sourcemay include a broadband plasma (BBP) light source. In this manner, the light generated by the light source and directed to the specimen may include broadband light. However, the light source may include any other suitable light source such as any suitable laser known in the art configured to generate light at any suitable wavelength(s). In addition, the laser may be configured to generate light that is monochromatic or nearly-monochromatic. In this manner, the laser may be a narrowband laser. The light source may also include a polychromatic light source that generates light at multiple discrete wavelengths or wavebands.

Light from optical elementmay be focused onto specimenby lens. Although lensis shown inas a single refractive optical element, in practice, lensmay include a number of refractive and/or reflective optical elements that in combination focus the light from the optical element to the specimen. The illumination subsystem shown inand described herein may include any other suitable optical elements (not shown). Examples of such optical elements include, but are not limited to, polarizing component(s), spectral filter(s), spatial filter(s), reflective optical element(s), apodizer(s), beam splitter(s), aperture(s), and the like, which may include any such suitable optical elements known in the art. In addition, the system may be configured to alter one or more elements of the illumination subsystem based on the type of illumination to be used for imaging.

The imaging subsystem may also include a scanning subsystem configured to change the position on the specimen to which the light is directed and from which the light is detected and possibly to cause the light to be scanned over the specimen. For example, the imaging subsystem may include stageon which specimenis disposed during imaging. The scanning subsystem may include any suitable mechanical and/or robotic assembly (that includes stage) that can be configured to move the specimen such that the light can be directed to and detected from different positions on the specimen. In addition, or alternatively, the imaging subsystem may be configured such that one or more optical elements of the imaging subsystem perform some scanning of the light over the specimen such that the light can be directed to and detected from different positions on the specimen. The light may be scanned over the specimen in any suitable fashion such as in a serpentine-like path or in a spiral path.

The imaging subsystem includes one or more detection channels. At least one of the detection channel(s) includes a detector configured to detect light from the specimen due to illumination of the specimen by the system and to generate output responsive to the detected light. The imaging subsystem shown inincludes two detection channels, one formed by collector, element, and detectorand another formed by collector, element, and detector. The two detection channels are configured to collect and detect light at different angles of collection. In some instances, both detection channels are configured to detect scattered light, and the detection channels are configured to detect light that is scattered at different angles from the specimen. However, one or more of the detection channels may be configured to detect another type of light from the specimen (e.g., reflected light).

In, both detection channels are shown positioned in the plane of the paper and the illumination subsystem is also shown positioned in the plane of the paper. Therefore, in this embodiment, both detection channels are positioned in (e.g., centered in) the plane of incidence. However, one or more of the detection channels may be positioned out of the plane of incidence. For example, the detection channel formed by collector, element, and detectormay be configured to collect and detect light that is scattered out of the plane of incidence. Therefore, such a detection channel may be commonly referred to as a “side” channel, and such a side channel may be centered in a plane that is substantially perpendicular to the plane of incidence.

Althoughshows an embodiment of the imaging subsystem that includes two detection channels, the imaging subsystem may include a different number of detection channels (e.g., only one detection channel or two or more detection channels). The detection channel formed by collector, element, and detectormay form one side channel as described above, and the imaging subsystem may include an additional detection channel (not shown) formed as another side channel that is positioned on the opposite side of the plane of incidence. Therefore, the imaging subsystem may include the detection channel that includes collector, element, and detectorand that is centered in the plane of incidence and configured to collect and detect light at scattering angle(s) that are at or close to normal to the specimen surface. This detection channel may therefore be commonly referred to as a “top” channel, and the imaging subsystem may also include two or more side channels configured as described above. As such, the imaging subsystem may include at least three channels (i.e., one top channel and two side channels), and each of the at least three channels is configured to collect light at different scattering angles than each of the other collectors.

As described further above, one or more of the detection channels may be configured to detect scattered light. Therefore, the imaging subsystem shown inmay be configured for dark field (DF) imaging. However, the imaging subsystem may also or alternatively include detection channel(s) that are configured for bright field (BF) imaging. Therefore, the imaging subsystems described herein may be configured for only DF, only BF, or both DF and BF imaging. Although each of the collectors are shown inas single refractive optical elements, each of the collectors may include refractive optical element(s) and/or reflective optical element(s).

The one or more detection channels may include any suitable detectors known in the art such as photo-multiplier tubes (PMTs), charge coupled devices (CCDs), and time delay integration (TDI) cameras. The detectors may also include non-imaging detectors or imaging detectors. If the detectors are non-imaging detectors, each of the detectors may be configured to detect certain characteristics of the light such as intensity but may not be configured to detect such characteristics as a function of position within the imaging plane. As such, the output that is generated by each of the detectors in each of the detection channels may be signals or data, but not image signals or image data. In such instances, a computer system may be configured to generate images of the specimen from the non-imaging output of the detectors. However, in other instances, the detectors may be configured as imaging detectors that are configured to generate imaging signals or image data. Therefore, the imaging subsystem may be configured to generate images in a number of ways.

Computer subsystemmay be coupled to the detectors of the imaging subsystem in any suitable manner (e.g., via one or more transmission media, which may include “wired” and/or “wireless” transmission media) such that the computer subsystem can receive the output generated by the detectors. Computer subsystemmay be configured to perform a number of functions using the output of the detectors as described further herein. Computer subsystemmay be further configured as described herein.

Computer subsystem(as well as other computer subsystems described herein) may also be referred to herein as computer system(s). Each of the computer subsystem(s) or system(s) described herein may take various forms, including a personal computer system, image computer, mainframe computer system, workstation, network appliance, Internet appliance, or other device. In general, the term “computer system” may be broadly defined to encompass any device having one or more processors, which executes instructions from a memory medium. The computer subsystem(s) or system(s) may also include any suitable processor known in the art such as a parallel processor. In addition, the computer subsystem(s) or system(s) may include a computer platform with high speed processing and software, either as a standalone or a networked tool.

If the system includes more than one computer system, then the different computer systems may be coupled to each other such that images, data, information, instructions, etc. can be sent between the computer systems. For example, computer subsystemmay be coupled to computer system(s)as shown by the dashed line inby any suitable transmission media, which may include any suitable wired and/or wireless transmission media known in the art. Two or more of such computer systems may also be effectively coupled by a shared computer-readable storage medium (not shown).

In an electron beam imaging subsystem, the energy directed to the specimen includes electrons, and the energy detected from the specimen includes electrons. In one such embodiment shown in, the imaging subsystem includes electron column, and the system includes computer subsystemcoupled to the imaging subsystem. Computer subsystemmay be configured as described above. In addition, such an imaging subsystem may be coupled to another one or more computer systems in the same manner described above and shown in.

As also shown in, the electron column includes electron beam sourceconfigured to generate electrons that are focused to specimenby one or more elements. The electron beam source may include, for example, a cathode source or emitter tip, and one or more elementsmay include, for example, a gun lens, an anode, a beam limiting aperture, a gate valve, a beam current selection aperture, an objective lens, and a scanning subsystem, all of which may include any such suitable elements known in the art.

Electrons returned from the specimen (e.g., secondary electrons) may be focused by one or more elementsto detector. One or more elementsmay include, for example, a scanning subsystem, which may be the same scanning subsystem included in element(s).

The electron column may include any other suitable elements known in the art. In addition, the electron column may be further configured as described in U.S. Pat. No. 8,664,594 issued Apr. 4, 2014 to Jiang et al., U.S. Pat. No. 8,692,204 issued Apr. 8, 2014 to Kojima et al., U.S. Pat. No. 8,698,093 issued Apr. 15, 2014 to Gubbens et al., and U.S. Pat. No. 8,716,662 issued May 6, 2014 to MacDonald et al., which are incorporated by reference as if fully set forth herein.

Although the electron column is shown inas being configured such that the electrons are directed to the specimen at an oblique angle of incidence and are scattered from the specimen at another oblique angle, the electron beam may be directed to and scattered from the specimen at any suitable angles. In addition, the electron beam imaging subsystem may be configured to use multiple modes to generate output for the specimen as described further herein (e.g., with different illumination angles, collection angles, etc.). The multiple modes of the electron beam imaging subsystem may be different in any output generation parameters of the imaging subsystem.

Computer subsystemmay be coupled to detectoras described above. The detector may detect electrons returned from the surface of the specimen thereby forming electron beam images of (or other output for) the specimen. The electron beam images may include any suitable electron beam images. Computer subsystemmay be configured to perform any step(s) described herein. A system that includes the imaging subsystem shown inmay be further configured as described herein.

are provided herein to generally illustrate configurations of an imaging subsystem that may be included in the system embodiments described herein. Obviously, the imaging subsystem configurations described herein may be altered to optimize the performance of the imaging subsystem as is normally performed when designing a commercial imaging system. In addition, the systems described herein may be implemented using an existing imaging subsystem (e.g., by adding functionality described herein to an existing inspection system) such as the tools that are commercially available from KLA Corp., Milpitas, Calif. For some such systems, the methods described herein may be provided as optional functionality of the imaging subsystem (e.g., in addition to other functionality of the imaging system). Alternatively, the imaging subsystem described herein may be designed “from scratch” to provide a completely new system.

Although the imaging subsystem is described above as being a light or electron beam imaging subsystem, the imaging subsystem may be an ion beam imaging subsystem. Such an imaging subsystem may be configured as shown inexcept that the electron beam source may be replaced with any suitable ion beam source known in the art. In addition, the imaging subsystem may include any other suitable ion beam system such as those included in commercially available focused ion beam (FIB) systems, helium ion microscopy (HIM) systems, and secondary ion mass spectroscopy (SIMS) systems.

The imaging subsystem may be configured to generate output, e.g., images, of the specimen with multiple modes. In general, a “mode” is defined by the values of parameters of the imaging subsystem used for generating images of a specimen (or the output used to generate images of the specimen). Therefore, modes may be different in the values for at least one of the parameters of the imaging subsystem (other than position on the specimen at which the output is generated). For example, the modes may be different in any one or more alterable parameters (e.g., illumination polarization(s), angle(s), wavelength(s), etc., detection polarization(s), angle(s), wavelength(s), etc.) of the imaging subsystem. The imaging subsystem may be configured to scan the specimen with the different modes in the same scan or different scans, e.g., depending on the capability of using multiple modes to scan the specimen at the same time.

In a similar manner, the electron beam subsystem may be configured to generate images with two or more modes, which can be defined by the values of parameters of the electron beam subsystem used for generating images for a specimen. Therefore, modes may be different in the values for at least one of the electron beam parameters of the electron beam subsystem. For example, different modes may use different angles of incidence for illumination.

The imaging subsystems described herein may be configured as an inspection system, a metrology system, and/or a defect review system. For example, the embodiments of the imaging subsystem shown inmay be modified in one or more parameters to provide different imaging capability depending on the application for which it will be used. In one such example, the imaging subsystem may be configured to have a higher resolution if it is to be used for metrology rather than for inspection. In other words, the embodiments of the imaging subsystem shown indescribe some general and various configurations for an imaging subsystem that can be tailored in a number of manners that will be obvious to one skilled in the art to produce systems having different imaging capabilities that are more or less suitable for different applications.

In this manner, the imaging subsystem may be configured for generating output that is suitable for detecting or re-detecting defects on the specimen in the case of an inspection system or a defect review system, respectively, and for measuring one or more characteristics of the specimen in the case of a metrology system. In an inspection system, computer subsystemshown inmay be configured for detecting defects on specimenby applying a defect detection method or algorithm to output generated by one or more of the detectors. In a defect review system, computer subsystemshown inmay be configured for re-detecting defects on specimenby applying a defect re-detection method to the output generated by detectorand possibly determining additional information for the re-detected defects using the output generated by the detector. In a metrology system, computer subsystemshown inmay be configured for determining one or more characteristics of specimenusing the output generated by detectorsand/or. The system may be further configured for detecting or re-detecting defects on the specimen, determining characteristics of the specimen, determining other information for the specimen, etc. as described further herein.

As noted above, the imaging subsystem is configured for scanning energy (e.g., light, electrons, etc.) over a physical version of the specimen thereby generating output for the physical version of the specimen. In this manner, the imaging subsystem may be configured as an “actual” subsystem, rather than a “virtual” subsystem. However, a storage medium (not shown) and computer system(s)shown inmay be configured as a “virtual” system. In particular, the storage medium and the computer system(s) may be configured as a “virtual” imaging system as described in commonly assigned U.S. Pat. No. 8,126,255 issued on Feb. 28, 2012 to Bhaskar et al. and U.S. Pat. No. 9,222,895 issued on Dec. 29, 2015 to Duffy et al., which are incorporated by reference as if fully set forth herein. The embodiments described herein may be further configured as described in these patents.

The system includes a computer subsystem, which may include any configuration of any of the computer subsystem(s) or system(s) described above. The computer subsystem is configured for separately aligning candidate alignment target images (also referred to herein simply as “candidate images”) in the images generated for the specimen to corresponding setup images for setup alignment targets selected to have mutually linearly independent vectors between locations of the setup alignment targets and a reference location. In this manner, QLIVBM utilizes a group of unique kernel images and their corresponding linear independent vectors determined during the setup of alignment and applies them during runtime alignment to greatly reduce the likelihood of false alignment and failed alignment site on wafer inspection platforms (and other platforms described herein).